Re: [PATCH] mm: update stale locking comment in do_anonymous_page()

[David Hildenbrand (Arm)]

On 3/31/26 16:29, Aditya Sharma wrote:
> The comment above do_anonymous_page() dates back to 2005 and describes
> the pre-per-VMA-lock world where mmap_lock was always held on entry.
> Since CONFIG_PER_VMA_LOCK was introduced (6.4), the fault handler now
> has a fast path that enters holding only a per-VMA read lock, with
> mmap_lock not held at all.
> 
> Update the comment to describe both entry contexts accurately.
> 
> Signed-off-by: Aditya Sharma <adi.sharma@xxxxxxxxxxx>
> ---
>  mm/memory.c | 22 +++++++++++++++++++---
>  1 file changed, 19 insertions(+), 3 deletions(-)
> 
> diff --git a/mm/memory.c b/mm/memory.c
> index c65e82c86..cc8dbbaea 100644
> --- a/mm/memory.c
> +++ b/mm/memory.c
> @@ -5210,9 +5210,25 @@ static struct folio *alloc_anon_folio(struct vm_fault *vmf)
>  }
>  
>  /*
> - * We enter with non-exclusive mmap_lock (to exclude vma changes,
> - * but allow concurrent faults), and pte mapped but not yet locked.
> - * We return with mmap_lock still held, but pte unmapped and unlocked.
> + * We enter in one of two locking contexts:
> + *
> + * 1) VMA lock path (FAULT_FLAG_VMA_LOCK set):
> + *    Entered holding a read lock on the faulting VMA (vma_start_read),
> + *    but NOT holding mmap_lock. This is the fast path introduced with
> + *    per-VMA locking (CONFIG_PER_VMA_LOCK). If this function cannot
> + *    complete the fault (e.g. needs to wait on I/O or encounters a
> + *    condition requiring the mm lock), it must return VM_FAULT_RETRY
> + *    and the caller will fall back to the mmap_lock path below.
> + *
> + * 2) mmap_lock path (FAULT_FLAG_VMA_LOCK not set):
> + *    Entered holding a non-exclusive (read) lock on mmap_lock, which
> + *    excludes VMA tree modifications but allows concurrent faults on
> + *    other VMAs. No per-VMA lock is held.
> + *
> + * In both cases, on entry the pte is mapped but not yet locked.
> + * On return, the pte is unmapped and unlocked, and whichever of
> + * the above locks was held on entry is still held (mmap_lock is
> + * not dropped, VMA read lock is not dropped, rather, the caller releases it).
>   */
>  static vm_fault_t do_anonymous_page(struct vm_fault *vmf)
>  {

Was this AI generated?

I don't think we want to have such a wall of text for each and every
function that can be called with VMA lock or mmap lock in read mode.

-- 
Cheers,

David